1. Field of the Invention
The present invention relates to electrostatic transducers and particularly to electrostatic transducers used for ultrasonic sound production and reception.
2. Prior Art
Referring to FIG. 1, shown therein is a general view illustrating an electrostatic transducer of the single type. In FIG. 1 the electrostatic transducer includes a disc type fixed electrode A, a vibrating member B placed on a surface of the fixed electrode A and a disc type movable electrode C provided on the surface of the vibrating membrane B. A power source E is applied between the movable electrode C and the fixed electrode A. The power source E generates, for example, an electric current having voltages of 100 to 200 volts and frequencies of 50 to 60 KHz. When electric signals are applied to the movable electrode C and the fixed electrode A from the power source E, the vibrating membrane B vibrates to generate ultrasonic waves. The directivity of the fixed electrode A and the vibrating membrane B can be approximated by the following equation based upon the directivity theory of circular piston sound sources since they are nearly flat vibrating surfaces: ##EQU1## In the above equations, .lambda. is the wavelength, d is the diameter of the sound source and angle .theta. means an opening angle with respect to the central axis of the transducer. In these equations it is assumed that .lambda. equals 8.7 mm and d equals 36 mm to calculate the directivity and the directivity is as shown in FIG. 2. That is, the side lobe is attenuated by only 16 dB. The application of a vibrator having such beam directivity to, for example, an ultrasonic switch leads to the generation of an identification area at a position which is useless as a result of the side lobe. In FIG. 3 is shown a transducer in which the peripheral area D of the vibrating surface is tilted forward (an output of the direction acoustic waves Y) to advance the phases of both sides of the vibrating surface which has been proposed by the inventor in order to eliminate the disadvantage or the large side lobe.
Referring to FIG. 3, the sandwich construction consisting of the fixed electrode 1, the vibrating membrane 2 and the movable electrode 3 is identical with that of the general electrostatic type ultrasonic transducer of the single type shown in FIG. 1; however, the central portion F of the fixed electrode 1 is flat and the periphery D of the fixed electrode 1 is conical. Furthermore, the fixed electrode 1 is provided with a large number of acoustic wave radiation holes 8 all over the surface thereof. The periphery of the vibrating membrane 2 and the movable electrode 3 are fixed between a fixing frame 6 and the end surfaces of the fixing stand 7. The output direction of acoustic waves is indicated by the arrow Y. At this time it should be pointed out that although the side lobe can be attenuated by 30 dB as is shown in FIG. 2 by the broken line, the resonance frequency is 80 KHz or more. The use of an ultrasonic transducer at such a high resonant frequency is undesirable since the attenuation of sound at such a high frequency is large. Suitable frequencies at which such a transducer is used are about 40 to 60 KHz.